Conventionally, in order to convey signals across an interface between two physical entities, where at least one of the physical entities is rotating, electrical slip rings are employed.
Conventionally, when the drive mechanism for the physical interface is provided by an off-axis gear mechanism, an optical rotating joint (ORJ), also known as a fibre optic rotary joint (FORJ) can additionally (or alternatively) be positioned along the axis of rotation to allow optical signals to be passed across the rotating interface. One type of ORJ is a single channel single-mode ORJ, i.e. one in which optical signals are transmitted across the rotating interface from one fixed single-mode optical fibre to one rotating single-mode optical fibre.
Conventionally however, when the when drive mechanism for the physical interface is provided by a substantially central drive shaft, this is considered to be an insurmountable obstacle, or at least a significant impediment, to the inclusion of an ORJ, at least one positioned along the axis of rotation. It is noted that, in contrast, slip rings are not affected by this as they are in an annular form, and hence do not require to be, and would tend not to benefit from, being positioned along the axis of rotation.